| One Stop Modernization | Zi Hui Yang | — | (415) 297 0624 | Electricians | San Lorenzo · Alameda County | — | — |
| One Touch Electric LLC | Julio Cesar Moreno | — | (510) 385 4663 | Electricians | Alameda · Alameda County | — | — |
| One Two Tree Care INC | La Mora Maribel De | — | (408) 898 0507 | Other | Fremont · Alameda County | — | — |
| One Two Trees Company | Christopher Matthew Altman | abc@abc.abc | (415) 757 7603 | Tree | Oakland · Alameda County | — | — |
| Onelawn Synthetic Turf | Darryl Frank Donovan | — | (650) 877 7770 | Other | San Lorenzo · Alameda County | — | — |
| Only One Construction Co | Juan Jr Garcia | — | (510) 750 5891 | GC, remodel, ADU | Hayward · Alameda County | — | — |
| Ontime Garage Door Service | Saed Murtada Abedalnabi | — | (510) 944 7070 | Other | Hayward · Alameda County | — | — |
| Onward Painting | Carlos Cesar Hernandez-Flores | onwardpainting@gmail.com | (510) 467 9150 | Windows, patio, painters | San Leandro · Alameda County | — | — |
| Onzone Heating & Cooling INC | Marat Gareev | — | (650) 698 7979 | HVAC | Pleasanton · Alameda County | — | — |
| Optimal Home & Design & Construction | David Leonard Horton | — | (510) 490 6100 | GC, remodel, ADU | Fremont · Alameda County | — | — |
| Orange Engineering LLC | Dehong Liu | — | (510) 735 6475 | GC, remodel, ADU | Fremont · Alameda County | — | — |
| Orange Gate Stucco | Orange Gate Stucco | — | (415) 378 9482 | Other | Hayward · Alameda County | — | — |
| Orea | Steve Kyoosung Bang | — | (925) 989 2171 | GC, remodel, ADU | Pleasanton · Alameda County | — | — |
| Orfil Construction | Espitia Orfil | — | (510) 224 0135 | GC, remodel, ADU | Hayward · Alameda County | — | — |
| Oriental Landscaping | Oriental Landscaping | — | (510) 261 1265 | Windows, patio, painters | Oakland · Alameda County | — | — |
| Orion Plumbing | Magallon Sergio Garcia | — | (510) 499 2988 | Other | San Lorenzo · Alameda County | — | — |
| Oro Construction | Gatica Perez Marcos | — | (650) 930 0901 | GC, remodel, ADU | Union City · Alameda County | — | — |
| Orozco Painting & Decorating | Orozco Painting & Decorating | info@rewopeus.com | (510) 395 0068 | Windows, patio, painters | Alameda · Alameda County | — | — |
| Orozcollective Construction INC | Jonathan David Gutierrez Orozco | — | (510) 463 7469 | GC, remodel, ADU | Oakland · Alameda County | — | — |
| Ortega's & Sons Construction INC | Mauro Bernadac Ortega | — | (209) 338 8205 | GC, remodel, ADU | Hayward · Alameda County | — | — |
| Ortega's Bay Area General Construction INC | Juan Alberto Lopez | — | (510) 586 6611 | GC, remodel, ADU | Hayward · Alameda County | — | — |
| Ortega's Construction | Marco Antonio Ortega Frias | ortegasconllc@gmail.com | (510) 388 7309 | GC, remodel, ADU | Oakland · Alameda County | — | — |
| Orth Construction INC | Michael Douglas Orth | — | (925) 454 9203 | GC, remodel, ADU | Livermore · Alameda County | — | — |
| Ortiz Construction | Sonia Tadea Ortiz Del Toro | interested@domainmarket.com | (510) 458 7710 | GC, remodel, ADU | Hayward · Alameda County | — | — |
| Ortiz Drywall | Ortiz Drywall | — | (408) 661 9324 | Other | Fremont · Alameda County | — | — |
| Ortiz Plumbing Drain Cleaning | Ortiz Plumbing Drain Cleaning | — | (925) 397 2271 | Other | Livermore · Alameda County | — | — |
| Ortiz Tile | Ortiz Tile | — | (925) 216 6427 | Other | Livermore · Alameda County | — | — |
| Ortzow Cydney Painting INC | Cydney Louise Ortzow | — | (510) 652 4034 | Windows, patio, painters | Oakland · Alameda County | — | — |
| Oscar Construction & Remodeling INC | William Foster Lowe | — | (510) 493 3456 | GC, remodel, ADU | Oakland · Alameda County | — | — |
| Oscar Moreno Tree Service | Moreno Antonio Oscar | — | (510) 586 9148 | Tree | Hayward · Alameda County | — | — |
| Oscar's Glass | Oscar's Glass | — | (510) 461 4917 | Windows, patio, painters | Castro Valley · Alameda County | — | — |
| Oscito | Adam Andreas Bustamante | — | (510) 565 5110 | Windows, patio, painters | Berkeley · Alameda County | — | — |
| Osman Construction INC | Abdelrahim Khalid Elnabulsi | — | (925) 444 5774 | GC, remodel, ADU | Castro Valley · Alameda County | — | — |
| Otai Construction INC | Christopher Al Malik Holt | — | (510) 506 1197 | GC, remodel, ADU | Oakland · Alameda County | — | — |
| Otter Construction | Jeffrey Cramer Perko | — | (925) 600 0220 | GC, remodel, ADU | Pleasanton · Alameda County | — | — |
| Our Handy Helper | Daniel Yun Pak | daniel@ourhandyhelper.com | (510) 207 0709 | GC, remodel, ADU | Castro Valley · Alameda County | — | — |
| Outdoor Environmental Services INC | Garrett Lee Koch | — | (925) 337 1271 | Windows, patio, painters | Livermore · Alameda County | — | — |
| Ouyang General Engineering INC | Chiu Kan Wong | — | (510) 213 9645 | HVAC | San Leandro · Alameda County | — | — |
| Over The Woodsman Builder INC | David Joseph Oberholzer | — | (510) 773 0180 | GC, remodel, ADU | Oakland · Alameda County | — | — |
| Owen | Kenneth James Owen | — | (510) 583 0984 | GC, remodel, ADU | Castro Valley · Alameda County | — | — |